The present invention relates generally to ion implantation systems, and more particularly to a mass analyzer and method associated therewith to mass analyze a ribbon shaped ion beam.
Ion implantation systems are used to dope semiconductors with impurities in integrated circuit manufacturing. In such systems, an ion source ionizes a desired dopant element, which is extracted from the source in the form of an ion beam. The ion beam is typically mass analyzed to select ions of a desired charge-to-mass ratio and then directed at the surface of a semiconductor wafer in order to implant the wafer with the dopant element. The ions of the beam penetrate the surface of the wafer to form a region of desired conductivity, such as in the fabrication of transistor devices in the wafer. A typical ion implanter includes an ion source for generating the ion beam, a beamline assembly including a mass analysis apparatus for mass resolving the ion beam using magnetic fields, and a target chamber containing the semiconductor wafer or workpiece to be implanted by the ion beam.
Typical ion beam implanters include an ion source for generating positively charged ions from ionizable source materials. The generated ions are formed into a beam and directed along a predetermined beam path to an implantation station. The ion beam implanter may include beam forming and shaping structures extending between the ion source and the implantation station. The beam forming and shaping structures maintain the ion beam and bound an elongated interior cavity or passageway through which the beam passes en route to the implantation station.
The mass of an ion relative to the charge thereon (e.g., charge-to-mass ratio) affects the degree to which it is accelerated both axially and transversely by an electrostatic or magnetic field. Therefore, the beam which reaches a desired area of a semiconductor wafer or other target can be made very pure since ions of undesirable molecular weight will be deflected to positions away from the beam and implantation of other than desired materials can be avoided. The process of selectively separating ions of desired and undesired charge-to-mass ratios is known as mass analysis. Mass analyzers typically employ a mass analysis magnet creating a dipole magnetic field to deflect various ions in an ion beam via magnetic deflection in an arcuate passageway that will effectively separate ions of different charge-to-mass ratios.
In order to achieve a desired implantation for a given application, the dose and energy of the implanted ions may be varied. The ion dose controls the concentration of implanted ions for a given semiconductor material. Typically, high current implanters are used for high dose implants, while medium current implanters are used for lower dose applications. The ion energy is used to control junction depth in semiconductor devices, where the energy levels of the beam ions determine the degree to which ions are implanted or the depth of the implanted ions within the semiconductor or other substrate material. The continuing trend toward smaller semiconductor devices requires a mechanism that serves to deliver high beam currents at low energies. The high beam current provides the necessary dose levels, while the low energy permits shallow implants.
In most prior art systems, ion implantation employed a pencil-type ion beam, wherein a relatively narrow beam was produced by the ion source and subjected to mass analysis, subsequent beam conditioning, and scanning before reaching the workpiece. Many present applications, however, wish to obtain shallow implants with a relatively high dopant concentration, for example, in shallow source/drain regions in semiconductor manufacturing. For shallow depth ion implantation, high current, low energy ion beams are desirable. In this case, the reduced energies of the ions cause some difficulties in maintaining convergence of the ion beam due to the mutual repulsion of ions bearing a like charge. High current ion beams typically include a high concentration of similarly charged ions that tend to diverge due to mutual repulsion. One solution to the above problem is to employ a ribbon-type ion beam instead of a pencil-type beam. One advantage of the ribbon-type beam is that the cross-sectional area of the beam is substantially larger than the pencil-type beam. For example, a typical pencil beam has a diameter of about 1-5 cm, wherein a ribbon-type beam may have a height of about 1-5 cm and a width of about 40 cm. With the substantially larger beam area, a given beam current has substantially less current density, and the beam a lower perveance. Use of a ribbon-type beam, however, has a number of unique challenges associated therewith.
In ion implantation systems, there remains a need for a ribbon beam ion implantation system that provides a uniform ribbon beam at the workpiece.
The following presents a simplified summary in order to provide a basic understanding of one or more aspects of the invention. This summary is not an extensive overview of the invention, and is neither intended to identify key or critical elements of the invention, nor to delineate the scope thereof. Rather, the primary purpose of the summary is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later.
The present invention is directed to an ion implantation system operable to generate a ribbon shaped ion beam and direct such a beam along a beamline path toward a workpiece. The implantation system comprises a mass analyzer operable to receive the ribbon shaped ion beam and deflect such beam along an arcuate path for mass analysis thereof. The mass analyzer of the present invention is configured to effectuate such mass analysis with minimal beam distortion.
In accordance with one aspect of the present invention, a ribbon beam ion implantation system is disclosed having a ribbon beam ion source, a mass analysis system, and an end station. The mass analysis system has an entrance end and an exit end, and is operable to deflect selected ions having a desired charge-to-mass ratio within the ribbon beam along a predetermined arcuate path from the entrance end to the exit end thereof. The mass analysis system further comprises a field clamp located proximate the entrance end, the exit end or both the entrance and exit ends. The field clamp is operable to substantially terminate fringing fields associated with the mass analysis system, thereby reducing beam distortion and improving beam uniformity.
In another aspect of the invention, the field clamp comprises a pair of ferromagnetic members extending in a direction associated with a width of the ribbon beam. When located on the entrance end of the mass analysis system, the ferromagnetic members, for example, iron, act to substantially prevent fringing fields from extending beyond the clamp, thereby reducing substantially an extent of the fringing fields at the entrance end of the mass analyzer. Consequently, ribbon beam distortion associated with fringing fields at the entrance end of the mass analyzer is reduced. Similarly, a pair of ferromagnetic members extending in the width direction of the ribbon beam may be located proximate the exit end of the mass analyzer, and operate to prevent fringing fields from extending from the exit mass analysis system beyond the clamp. Accordingly, the extent of the fringing fields from the exit end of the mass analysis system is greatly reduced, thereby reducing beam distortion associated therewith and providing improved ribbon beam uniformity.
In yet another aspect of the present invention, the mass analysis system comprises a pair of coils extending arcuately and defining the beamline path therebetween. A first coil of the pair resides above the beam and extends along the entire width thereof, while the second coil of the pair resides below the beam and also extends along the entire beam width. By generating a current in the coils, a dipole magnetic field is generated between the coils that operates to deflect the desired ions within the beam along the arcuate path. While the dipole field between the coils along the beamline path is substantially uniform, fringing fields extend from the entrance end and the exit end due to, among other things, a magnetic field gradient along the beamline path. Such fringing fields may have transverse, non-uniform components along the width of the beam and operate to provide beam distortion along the width. The field clamps of the present invention operate to substantially terminate the fringing fields proximate the entrance and exit ends, respectively. Thus the ribbon beam traveling along the beamline path is exposed to fringing fields for a substantially smaller duration of time and thus distortion associated with such fringing fields is substantially reduced.
In still another aspect of the present invention, one or more pairs of secondary coils are employed in conjunction with the primary coils, wherein the secondary coils also extend along the arcuate passageway. The one or more pairs of secondary coils may overlie the primary coils or may extend along sidewall portions of the guide to provide further magnetic field compensation. In one example, such compensation is a function of a beam uniformity determination at the workpiece, wherein such compensation operates to compensate for distortion of the ribbon beam due to, for example, fringing fields or other factors despite the substantial reduction of the impacts of such fringing fields by the field clamps.
In accordance with still another aspect of the present invention, a method of mass analyzing a ribbon beam is disclosed. The method comprises generating a ribbon shaped ion beam, and sending such beam along a beamline path. A dipole magnetic field is generated along the beamline path to deflect selected ions in the ribbon beam upon a predetermined arcuate path according to a desired charge-to-mass ratio. Fringing fields associated with at least an entrance or exit portion of the dipole magnetic field are reduced to prevent beam distortion associated therewith. In one example, the fringing fields are reduced by providing a field clamp proximate the entrance or exit end of the dipole magnetic field such that a ribbon beam traveling along the beamline path is exposed to such fringing fields for a shorter duration of time, thereby reducing beam distortion associated therewith and providing improved ribbon beam uniformity.
To the accomplishment of the foregoing and related ends, the following description and annexed drawings set forth in detail certain illustrative aspects and implementations of the invention. These are indicative of but a few of the various ways in which the principles of the invention may be employed. Other aspects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.